This invention relates generally to the thermal protection of AC induction motors, and in particular, to a device and method for estimating the resistance of a stator winding of an AC induction motor in order to monitor the temperature of the winding.
As is known, organic materials used for stator winding insulation in an AC induction motor are subject to deterioration from excessive heat generated in the winding during operation of the motor. While it can be appreciated that the stator winding and the insulation therefore are designed not to deteriorate during normal operation of the motor, there are situations where the temperature of the winding can exceed its maximum limit thereby causing deterioration of the insulation. These situations on the motor include transient overload conditions such as motor stall, jam, and start-up; running overload conditions such as an overloaded motor; and abnormal cooling situations wherein the flow of the cooling air is accidentally obstructed. Insulation failure of the stator winding degrades the performance of the motor and eventually leads to motor failure. Hence, thermal protection of a motor is fundamental to proper operation of the motor.
Typically, stator winding temperature estimation for thermal protection of a motor is based on the thermal heat transfer model of an induction machine. Thermal devices such as bimetallic strips or eutectic melting alloys have been used for protecting the stator windings of induction machines. In addition, software programs, which estimate the internal temperature of the induction machine using estimated power losses and a microprocessor embedded thermal heat transfer model of the induction machine, have become commercially available. These software programs use approximated thermal models and assume that the stator and rotor resistance Rs and Rr, respectively, and the thermal parameters are fixed. However, it has been found that use of approximated thermal models may result in unacceptable temperature estimates when a dramatic change in the thermal situation of the induction machine occurs.
Alternatively, a stator resistance estimation technique may be used for the purpose of monitoring the temperature of the stator winding. As is known, resistance is a direct indicator of temperature. Consequently, the temperature of the stator winding of the induction machine can be determined from the resistance thereof. Further, since resistance-based temperature estimation is also capable of detecting the reduced cooling ability of the induction machine, resistance based temperature estimation provides significant advantages over conventional thermal model based methods.
An example of a device and method for estimating the resistance of a stator winding of an induction machine is shown in Paice, U.S. Pat. No 4,083,001. The device includes an asymmetric resistance device taking the form of a circuit having two parallel branches with diodes connected with opposite polarity in the two branches so that the device is conductive in both directions. The asymmetric characteristics are obtained by using different numbers of diodes in the two branches so that the resistances are different. During operation of the AC motor, a small direct current component is introduced. The resistance of the stator winding is then determined from measurements of the direct current component and the corresponding voltage. As heretofore described, since resistance is a function of temperature, the temperature of the stator winding may be monitored.
While functional for its intended purpose, the device and method disclosed in the Paice ""001 patent have certain disadvantages. For example, a user has no control over the magnitude of the DC current and voltage supplied to the AC motor. As such, the functionality of the device and method shown in the Paice ""001 patent may vary depending on the size and the value of the resistance of the AC motor. Further, the device disclosed in the Paice ""001 patent is operatively connected to the AC motor at all times. However, the change in resistance of the stator winding of an AC motor changes slowly over time. Hence, it is unnecessary for a user to constantly measure the resistance of the stator winding. Further, the DC offset generated by the device shown in the Paice ""001 patent generates a torque pulsation in the AC motor that is proportional to the DC current and voltage bias injected thereto. As a result, a significant amount of energy may be wasted through the use of the device and method disclosed in the Paice ""001 patent.
Therefore, it is a primary object and feature of the present invention to provide a device and method for estimating the resistance of a stator winding of an AC induction motor which is simple and inexpensive to utilize.
It is a further object and feature of the present invention to provide a device and method for estimating the resistance of a stator winding of an AC induction motor which dissipates small amounts of energy during use.
It is a still further object and feature of the present invention to provide a device and method for estimating the resistance of a stator winding of an AC induction motor which injects a DC voltage and current bias to the AC induction motor only during predetermined time periods.
In accordance with the present invention, a device is provided to estimate the resistance of a stator winding of an AC motor. The AC motor has a motor terminal connectable to an AC source for providing voltage and current to the AC motor. The device includes a resistor circuit having a first input node and a second output node. The input node is connectable to the AC source and the output node is connectable to the motor terminal. A switch is connected in parallel with the resistor circuit. The switch is movable between the first open position and a second closed position. A voltmeter is provided for measuring the voltage drop across the resistor and an ammeter is provided for measuring the DC current supplied to the motor terminal. A digital signal processor is operatively connected to the switch, the voltmeter and the ammeter. The digital signal processor controls the movement of the switch between the open and closed positions so as to inject a DC voltage and current bias to the AC motor. In addition, the digital signal processor estimates the resistance of a stator winding in response to the DC voltage and current bias. It is contemplated that the switch be either a MOSFET or an IGBT.
In accordance with a still further aspect of the present invention, a method is provided for estimating the resistance of a stator winding of an AC motor. The AC motor has a motor terminal connectable to an AC source for supplying voltage and current to the AC motor. The method includes the steps of connecting a resistor circuit and a switch in parallel and providing the same as a DC injection circuit. The motor terminal is interconnected to the AC source with the DC injection circuit. The switch is opened during a first half cycle of each complete alternation of the current supplied to the AC motor and closed during a second half cycle of each complete alternation of the current supplied to the AC motor so as to inject a DC voltage and current bias to the AC motor. The resistance of the stator winding is estimated in response to the DC voltage and current bias.
In a first embodiment, the DC voltage is measured between the motor terminal and a neutral point during a sample time period and is provided as the line-neutral voltage. The DC current injected to the AC motor is also measured during the sample time period and provided as the line current. The resistance of the stator winding is estimated according to the expression:
Rs=vas/ias
wherein Rs is the estimated resistance of the stator winding; vas is the line-neutral voltage; and ias is the line current.
Alternatively, in a second embodiment, the DC voltage drop across the switch is measured during a sample time period and provided as the switch voltage. As such, the resistance of the stator winding may be estimated according to the expression:
Rs=(2*vsw)/(3*ias)
wherein Rs is the estimated resistance of the stator winding; vsw is the switch voltage; and ias is the line current.
In a third embodiment, the AC motor may include a second motor terminal connected to the AC source by a line in order to provide voltage and current to the AC motor. The DC voltage drop between the first and second motor terminals is measured during a sample time period and provided as the line-line voltage. As such, the resistance of the stator winding may be estimated according to the expression:
Rs=(2*vab)/(3*ias)
wherein Rs is the estimated resistance of the stator winding; vab is the line-line voltage; and ias is the line current.
It is contemplated that the resistor circuit include a resistor and the switch be either a MOSFET or an IGBT. The opening and closing of the switch occur at zero crossings of the current supplied to the AC motor by the AC source. The opening of the switch during the first half cycle of each complete alternation of the current supplied to the AC motor and the closing of the switch during the second half cycle of each complete alternation of the current supplied to the AC motor occur for predetermined time periods. It is contemplated that the first half cycle be the positive half cycle of each complete alternation of the current supplied to the AC motor and the second half cycle be the negative half cycle of each complete alternation of the current supplied to the AC motor.
In accordance with the further aspect of the present invention, a method is provided for estimating resistance of a stator winding of a three phase, AC motor. Each phase of the AC motor is connectable to an AC source for supplying voltage and current to the AC motor. The method includes the steps of providing a resistor circuit and a switch in parallel and providing the same as a DC injection circuit. A first phase of the AC motor is interconnected to the AC source by the DC injection circuit. A DC voltage and current bias is injected to the AC motor with the DC injection circuit. The resistance of the stator winding is estimated in response to the DC voltage and current bias.
The step of injecting the DC voltage and current bias to the AC motor includes the steps of opening the switch during the first half cycle of each complete alternation of the current supplied to the AC motor and closing the switch during the second half cycle of each complete alternation of the current supplied to the AC motor. The DC voltage and the current bias to the AC motor is injected for a predetermined period of time.
In a first embodiment, the DC voltage is measured between the first phase of the AC motor and a neutral point during a portion of the predetermined time period and is provided as the line-neutral voltage. The DC current injected to the first phase of the AC motor is also measured during the sample time period and provided as the line current. The resistance of the stator winding is estimated according to the expression:
Rs=vas/ias
wherein Rs is the estimated resistance of the stator winding; vas is the line-neutral voltage; and ias is the line current.
Alternatively, in a second embodiment, the DC voltage drop across the switch is measured during a portion of the predetermined time period and provided as the switch voltage. As such, the resistance of the stator winding may be estimated according to the expression:
xe2x80x83Rs=(2*vsw)/(3*ias)
wherein Rs is the estimated resistance of the stator winding; vsw is the switch voltage; and ias is the line current.
In a third embodiment, the AC motor may include a second motor terminal connected to the AC source by a line for providing voltage and current to the AC motor. The DC voltage drop between a first and a second phase of the AC motor is measured during a portion of the predetermined time period and provided as a line voltage. As such, the resistance of the stator winding may be estimated according to the expression:
Rs=(2*vab)/(3*ias)
wherein Rs is the estimated resistance of the stator winding; vab is the line-line voltage; and ias is the line current.